1) Field of the Invention
The present invention relates to an image forming apparatus and a method of correcting timing for generating laser beams that takes accounts for fluctuations in temperature.
2) Description of the Related Art
An image forming apparatus writes image information on a photoreceptor by scanning the photoreceptor with laser beams. The laser beams are deflected, for scanning the photoreceptor, with a polygon mirror and a fθ lens. Sometimes an achromatic fθ lens is used to suppress variations in writing magnification as disclosed, for example, in Japanese Patent Application Laid Open (JP-A) No. SHO 62-254110 (see pages 1 and 2, and FIG. 1). However, if the achromatic fθ lens is made of glass, it increases the weight and cost of the apparatus. Therefore, to realize weight and cost reduction, fθ lenses made of plastic are becoming popular.
However, the plastic lenses have a problem that they deform due to change in ambient temperature and change in temperature inside the apparatus. Moreover, the polygon mirrors have a problem that their figure tolerance changes due to change in ambient temperature and change in temperature inside the apparatus. If the lens deforms or the figure tolerance of the polygon mirror changes, the precision of the image formation degrades, because, it shifts the scanning position. The effect becomes prominent when the photoreceptor is scanned with a plurality of laser beams, because, there occurs color misalignment due to a magnification error in a main scanning direction and each magnification error of the beams.
One approach to solve the above mentioned problem is disclosed, for example, in JP-A No. HEI 9-58053 (see pages 2 to 5, and FIG. 1). FIG. 8 is a conceptual diagram of an optical writing system of this image forming apparatus. FIG. 8 illustrates only one optical writing system although there are a plurality of optical writing systems.
The image forming apparatus includes a laser diode 101, which emits a laser beam, and a laser driving circuit 109, which drives the laser diode. The laser beam emitted from the laser diode 101 is deflected by a polygon mirror 102 towards the photoreceptor 103. Laser detecting sensors 105 and 106 are provided along the main scanning direction. When the laser detecting sensor 105 detects a laser beam, it outputs a detection signal det 1, and when the laser detecting sensor 106 detects a laser beam, it outputs a detection signal det 2, to a write clock generating circuit 107. The write clock generating circuit 107 measures a number of counts of a predetermined clock (hereinafter, also referred to as “a number of clocks”) during a period between detection of a detection signal det 1 and detection of a subsequent detection signal det 2. The write clock generating circuit 107 compares the value of the clocks obtained by the measurement (hereinafter, “measured value”) with a reference value, and outputs a write clock CLK0 of a frequency such that the measured value substantially coincides with the reference value. The laser driving circuits are activated based on this write clock CLK0. This eliminates the effect of the change in temperature on the scanning.
Although not shown, when there are plurality of beams, there is one laser driving unit, one laser diode, two laser detecting sensors, and one write clock frequency CLK0 corresponding to each laser beam.
However, the amount of calculations required to generate the write clocks of an appropriate frequency is vast. As a result, since a phase-locked loop (PLL) circuit is generally used to generate the write clocks, desired write clocks are not output. This causes color misalignment, i.e., the images produced by each laser beam shift from theoretical positions and therefore the images are not superposed properly.